An unpredictable Olympic tennis tournament claimed its biggest scalp of all as Serena Williams crashed out in the third round of the women's singles.

The world number one appeared to have a shoulder problem and was close to tears as she lost 6-4 6-3 to Ukraine's Elina Svitolina.

Serena and sister Venus are the most successful players in Olympic tennis history with four gold medals each but Rio has been dreadful for the family.

Venus lost in the opening round of the singles to Kirsten Flipkens while the sisters lost their first ever Olympic doubles match on Sunday.

Serena won gold in both events in London four years ago and appeared to have put doubts about her form to bed by winning her 22nd grand slam singles title at Wimbledon last month.

The 34-year-old's exit means Angelique Kerber and Madison Keys are the only top-eight seeds remaining of what had appeared a very strong field.